Composite aluminum-containing workpieces

ABSTRACT

Processes and compositions for soldering aluminum-containing workpieces wherein the solder contains components which lower the surface tension and the viscosity of the molten solder, and which lower the interfacial tension between the solder and the aluminum-containing workpieces. Useful components are bismuth, strontium, barium and antimony. The solder may be used with or without flux.

CROSS-REFERENCE TO RELATED APPLICATION

This is a division of application Ser. No. 729,039 filed Oct. 4, 1976,now U.S. Pat. No. 4,121,750 which is a continuation of Ser. No. 368,627filed June 11, 1973, now abandoned, which is a continuation-in-part ofSer. No. 98,173, filed Dec. 14, 1970, now abandoned.

BACKGROUND OF THE INVENTION

In the soldering process, in contrast to what happens during welding,the added metal must melt, but the workpiece itself must not melt. Forthis reason, the melting region of the solder and the work temperaturemust lie beneath the melting point of the workpiece. The term"soldering" is commonly used to refer to processes for joining togethermetal surfaces with an alloy which may be a soft or a hard solder, andthe term is used herein in this broad sense. When a hard solder is used,the process, which is generally carried out at a temperature above 450°C., is also referred to and known as brazing and the hard solder isreferred to as a brazing alloy. Soft or common solder is used attemperatures below 450° C. and hard solder is relatively infusible ascompared with soft solder.

In the soldering of aluminum and aluminum alloys, a serious problemexists by reason of the fact that a very resistant layer of oxide ispresent on the surface and prevents the molten solder from wetting themetal therebeneath. Although this oxide layer is very thin, it isnevertheless dense and stable, and after removal, a new film reformsspontaneously, even during the soldering process, so long as theoperation is carried out in an oxidizing atmosphere such as air. It hashitherto been necessary for the soldering of aluminum-containingworkpieces to remove the oxide layer from the region where the solderingis to take place, either by mechanical or by chemical means. One suchmethod has been friction soldering, a special technique in which softsolder is applied and the oxide layer removed together with the moltensolder. In a variation of this technique, ultrasonic soldering has beenattempted but such technique has not come into common use.

Apart from such special processes, the removal of the oxide layer hasbeen carried out by means of fluxes which, in addition, prevent theformation of a new oxide layer. The fluxes suitable for use in thesoldering of aluminum, in general, are chlorides or fluorides and, forsoft soldering, pure organic compounds are also used. All fluxes havethe disadvantage that they lead to corrosion, most leading to verystrong corrosion, and consequently residues must be completely removed.Moreover, there is always the danger of inclusion of flux in the solderjoint. Residues and inclusions of flux damage the corrosion resistanceof the solder portion, especially when dampness has access to thesoldered joint. Removal of solder residues is costly and consequently isgenerally not complete. It is for such reasons that research has beendirected to solders and especially hard solders which can operatewithout fluxes.

The difficulties described above become especially significant when thestandards of strength and corrosion resistance for a soldered joint areconsidered. Solders previously used for soldering aluminum without fluxhave consequently not matured to actual practice or commercialproduction. As examples of processes that have been proposed in U.S.patents and publications, it has been recommended that the oxide layerbe removed either through exothermic reaction or by reduction at thelocation where the joint is to be made, and this be carried out eitherin a high vacuum at an absolute pressure of about 10⁻⁶ torr, or with acombination of a reduction process in a vacuum at an absolute pressureof about 10⁻⁴ torr. In such processes, the solder, as well as anyadditional components, are generally added in the form of a mixture ofpowder.

In addition to the very dubious efficiency of the removal of oxide bythe foregoing methods, these methods also have the disadvantages thatthe powders must be protected from oxidation during their preparationand stratification of the components must be prevented when the powderis applied to the workpiece, making such processes unsuitable forquantity production of aluminum workpieces. Also, large-scale use ofhigh vacuum for soldering requires high costs for preparation andoperation. Consequently, such processes can be used in only very specialcases, such as for the manufacture of parts to be used with reactors orrockets.

As a general rule, the solders used for soldering aluminum-containingworkpieces have the disadvantage of relatively high viscosity andsurface tension, as well as poor wetting characteristics. This isparticularly true for hard solders which, as state above, have specialsignificance. Such hard solders are usually of the aluminum-silicon(Al-Sl) type which may also contain copper (Cu), magnesium (Mg), nickel(Ni), zinc (Za), tin (Sn) and cadmium (Cd). In addition to thecommercial soft solders, there are zinc-aluminum (Zn-al) solders which,according to the particular compositions and thereby the operatingtemperature, can be rated either as hard or as soft solders.

In order to avoid the difficulties of high viscosity and high surfacetension, the flux contains in part additives in the form of zinc saltswhich produce a metal layer on the clean basic metal surface andincrease the wetting by the solder at the point of joining.

A method for the preparation of solder to be used for soldering purealuminum or nearly pure aluminum was disclosed in German Pat. No.66,398, in which a substantial portion of the pure aluminum was meltedand then the surface of the molten metal was covered with a layer ofphosphoric acid, sodium bisulfate, fluorine compounds, or other acidicsalts, and finally, to the molten metal was added a small quantity ofcopper and tin; or copper bismuth, zinc and tin; or copper, antimony,bismuth and zinc; or copper, bismuth, antimony and tin. Although thispatent was issued in 1891, this process has never come into use. Such asolder would not be practical owing to the small difference intemperature between the melting point of the solder and of theworkpiece, and the only reason why it was considered at all, was due tothe fact that the melting point of aluminum was thought to be 800° C. atthat time (page 1, left-hand column, paragraph 2 of that patent), whenit is actually 659.7° C.

As was stated above, the fundamental problem in the soldering ofaluminum lies in the fact that the work-piece is always covered with askin of aluminum oxide which is only poorly wetted by the molten solder.It was therefore previously concluded that, for the achievement ofsatisfactory soldered joints on aluminum-containing workpieces, theoxide skin must be broken down and removed.

SUMMARY OF THE INVENTION

In contrast to the view previously adopted with respect to the problemof soldering aluminum-containing workpieces, we have found that thewettability of the region to be soldered can be greatly improved by theaddition of metallic elements other than those which have beenpreviously used in solders for this purpose, which added elements reducethe viscosity and the surface tension of the molten solder and similarlyreduce the interfacial tension between the molten solder and theworkpiece.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Our researches have shown that a strong reduction of the viscosity andthe surface tension of the molten solder, as well as of the interfacialtension between the molten solder and the workpiece, can be achieved bythe addition of bismuth and/or strontium and/or barium and/or antimonyin quantities of 0.01 to 10%, and preferably 0.05 to 2%, where thesesolder are free of silicon. However, it is also possible to prepare asuitable solder which contains at least 6% silicon as well as bismuthand/or strontium and/or barium and/or antimony in quantities of 0.01 to10%, and preferably 0.05 to 2%.

Suitable solders for use in the soldering processes of the presentinvention may be of the following alloy types:

(a) Between 2 and 26% by weight of aluminum with the remainder beingzinc;

(b) Between 6 and 20% by weight of silicon, with optionally variousquantities of other alloying elements such as copper, magnesium, nickel,zinc, tin, and cadmium with the remainder being aluminum.

(c) Between 26 and 45% by weight of aluminum, and optionally up to 1% byweight of manganese, with the remainder being zinc.

Solders of all three of the foregoing types may, and preferably will,contain between 0.01 and 10% by weight of at least one of the metals ofthe group consisting of bismuth, strontium, barium, and antimony, ashereinbefore stated.

Solders of type a are soft solders whereas solders of types b and c arehard solders. Specific examples of solder compositions of each of thesetypes are indicated in the Examples hereinafter.

We have discovered that solders such as are disclosed herein aresuitable for use in fluxless soldering of aluminum when such solderingis carried out in a nonoxidizing atmosphere, or in an atmospherecontaining only a small proportion of oxygen. A suitable method is touse an inert solder-blanketing gas such as argon of welding grade,nitrogen of commercial purity, or ammonia. Another method is to work ina low vacuum having for instance an absolute pressure of between 10⁻¹and 10 torr. Vacua in the range of absolute pressures between 10⁻¹ and10⁻⁶ torr as have been proposed for previously described methods offluxless soldering, are definitely unnecessary for the processes of thepresent invention. More than the conventional precautions, such as thecareful cleaning and drying of the area, which have hitherto beennecessary, are not required in the soldering processes of the presentinvention. As with fluxless solders, the present process has theadvantage that acid treatment to remove flux residues is not necessary.

The above-described reduction of the surface tension and viscosity, andthe thereby improved wetting properties are also, naturally, anadvantage when the soldering is carried out with an added flux. Theaddition of special materials to the flux to improve its wettingproperties can be omitted when the solders described herein are used.Moreover, the composition of the flux can be altered in such a way thatadvantages are achieved in the removal of the flux residues. Soldershaving the compositions specified herein can be prepared and used in allof the conventionally used forms, such as wires, bars, shaped pieces,foils and solder plated sheets.

Examples of typical soldering compositions which can be used in theprocesses in accordance with the present invention are the following:

    ______________________________________                                        EXAMPLE 1                                                                     ______________________________________                                                  Si      11.7%                                                                 Mn      0.09%                                                                 Mg      0.03%                                                                 Fe      0.35%                                                                 Ti      0.02%                                                                 Cu      0.02%                                                                 Zn      0.05%                                                                 Bi      0.15%                                                                 Remainder aluminum                                                  ______________________________________                                        EXAMPLE 2         EXAMPLE 3                                                   ______________________________________                                        Si        12.3%        Si        7.2%                                         Mn        0.07%        Mn        0.07%                                        Mg        0.02%        Mg        0.03%                                        Fe        0.25%        Fe        0.63%                                        Ti        0.01%        Ti        0.04%                                        Cu        0.01%        Cu        0.18%                                        Zn        0.04%        Zn        0.11%                                        Bi        0.50%        Bi        1.95%                                        Sb        0.31%        Remainder aluminum                                     Remainder aluminum                                                            ______________________________________                                        EXAMPLE 4         EXAMPLE 5                                                   ______________________________________                                        Si        8.1%         Si        15.8%                                        Mn        0.05%        Mn        0.03%                                        Mg        0.02%        Mg        0.04%                                        Fe        0.49%        Fe        0.41%                                        Ti        0.02%        Ti        0.03%                                        Cu        0.13%        Cu        0.11%                                        Zn        0.09%        Zn        0.18%                                        Ba        2.13%        Sb        1.11%                                        Sr        0.76%        Ba        1.37%                                        Remainder aluminum Remainder aluminum                                         ______________________________________                                        EXAMPLE 6        EXAMPLE 7                                                    ______________________________________                                        Si        16.5%        Zn        95.70%                                       Mn        0.03%        Al        3.95%                                        Mg        0.03%        Bi        0.34%                                        Fe        0.43%        Impurities                                                                              0.01%                                        Ti        0.03%                                                               Cu        0.13%                                                               Zn        0.17%                                                               Ba        1.78%                                                               Remainder aluminum                                                            ______________________________________                                        EXAMPLE 8         EXAMPLE 9                                                   ______________________________________                                        Zn        94.23%       Zn        67.59%                                       Al        5.10%        Al        30.40%                                       Bi        0.18%        Sb        1.50                                         Ba        0.47%        Mn        0.35%                                        Impurities                                                                              0.02%        Impurities                                                                              0.10%                                        ______________________________________                                        EXAMPLE 10                                                                    ______________________________________                                                  Zn      68.10%                                                                Al      28.73%                                                                Sr      2.67%                                                                 Mn      0.33%                                                                 Impurities                                                                            0.09%                                                       ______________________________________                                    

It will be noted that the compositions of Examples 7 and 8 are softsolders of type a.

The solders whose compositions are specified in the foregoing exampleswere used for soldering pure aluminum and aluminum-manganese sheets. Thesamples were heated in an electrically heated oven. In all cases,soldering of aluminum workpieces was carried out without fluxes in aprotective gas atmosphere with a gas such as argon or nitrogen. Thesoldering temperatures for the hard aluminum-silicon solders are between590° and 605° C., for the zinc-aluminum hard solders (type c) between530° and 545° C., and for the zinc-aluminum soft solders (type a) arebetween 400° and 415° C. The soldering time amounted to about 2 minutes.In all cases, satisfactory soldered joints were achieved.

In contrast thereto, attempts to prepare soldered joints withaluminum-containing workpieces using the commercial solders L-AlSi 12and ZnAl 4 failed when no flux was used. The composition of the L-AlSi12 solder in percentages by weight is as follows:

    ______________________________________                                                    11-13.5                                                                   Mn  0.1                                                                       Fe  0.4                                                                       Ti  0.03                                                                      Cu  0.03                                                                      Zn  0.07                                                                     Other individual components 0.03                                              Total of other components 0.05                                                Al remainder                                                           ______________________________________                                    

The composition of the ZnAl 4 solder in percentages by weight is asfollows:

Al 3.9

Impurities 0.04 (Cu, Mg, Si, Fe, B, Cd, Pb)

Zn remainder

Both solders were produced by Vereinigte Aluminimum-WerkeAktiengesellschaft, Bonn.

Vibration of the sample during the soldering process did not improve theresults.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can by applying current knowledgereadily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this inventionand, therefore, such adaptations should and are intended to becomprehended within the meaning and range of equivalence of thefollowing claims.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims.

We claim:
 1. A bonded article comprising a pair of spaced apart workpieces consisting of aluminum and bearing an aluminum oxide surface layer,an aluminum silicon filler metal interposed between said oxide surface layers to form a joint between said workpieces, said filler metal consisting essentially of aluminum containing from 6 to 20% by weight of silicon and from 0.01 to 10.0% by weight of said filler metal of at least one substance selected from the group consisting of bismuth, antimony, strontium and barium remainder aluminum and impurities, said filler metal having a melting temperature substantially below that of aluminum and said joint being free of any fluoride or chloride flux residue.
 2. A clad brazing material comprising a core consisting essentially of aluminum, a flux-free filler metal clad on the surface of said core and consisting essentially of 6 to 20% by weight of silicon and from 0.01 to about 10% by weight of at least one substance selected from the group consisting of bismuth, antimony, strontium and barium, the balance aluminum and impurities, the viscosity and surface tension of said filler metal in the molten condition being less than the viscosity and surface tension of said molten filler metal in the absence of said substance and the interfacial tension between said filler metal and an aluminum workpiece being lower than the interfacial tension in the absence of said substance, and said filler metal having a melting temperature substantially below that of aluminum.
 3. An article comprising a first aluminum alloy, said first alloy consisting essentially of from 6 to 20% by weight of silicon and from 0.01 to 10% by weight of at least one substance selected from the group consisting of bismuth, antimony, strontium, and barium, the remainder aluminum and its alloys, the surface tension of said first alloy in the molten condition being less than the surface tension of said molten alloy in the absence of said substance, andworkpieces comprising aluminum and aluminum alloys having melting points higher than that of said first alloy, said workpieces joined together by said first alloy in a flux-free bond.
 4. A clad article comprising a core selected from aluminum and its alloys:a cladding on said core, said cladding comprising a flux-free filler metal consisting essentially of from about 6 to about 20% by weight of silicon, from about 0.01 to 10% by weight of at least one substance selected from the group consisting of bismuth, strontium, barium and antimony, remainder aluminum and its alloys and said cladding having a melting temperature substantially below that of said core materials.
 5. The bonded article of claim 1 wherein said substance comprises bismuth.
 6. The bonded article of claim 5 wherein said bismuth is present in the range between about 0.05 to 2% by weight.
 7. The clad brazing material of claim 2 wherein said substance comprises bismuth.
 8. The clad brazing material of claim 48 wherein said bismuth is present in the range from about 0.05 to about 2% by weight. 